In many wireless communications systems the CSI (Channel-State Information) feedback is crucial for obtaining good performance. Reference signals (RS) are transmitted which are used to estimate the channel state. The feedback reporting typically includes a CQI (Channel-Quality Indicator) and RI (Rank Indicator) value. More detailed reports may include frequency-selective CQI and/or PMI (Pre-coding Matrix Indicator) values.
The 3rd-Generation Partnership Project (3GPP) so-called Long-Term Evolution (LTE) systems support CSI-reporting schemes that rely on the reference symbols being transmitted periodically, where the cell-specific reference symbols (CRS) are sent every subframe while CSI-RS can be sent with a larger periodicity. User equipments (UEs) using transmission mode 10 (TM10) can rely on CSI-RS resources while other UEs typically use the CRS at least for interference measurements.
TM10 UEs can be configured to report CSI for multiple CSI-processes, which each may have different CSI-measurement resources. A CSI-measurement resource can consist of a CSI-RS resource and a CSI-IM (CSI interference measurement) resource. Both the CSI-RS and the CSI-IM resources are characterized (among other things) by a subframe configuration. The subframe configuration specifies a subframe periodicity and a subframe offset that specify for the UE at which time instances the respective measurement resources are available.
License-Assisted Access via LTE (LAA-LTE) has recently been proposed as a technology for co-existence on unlicensed carriers with, e.g., WiFi. On an unlicensed carrier all data transmissions are to be controlled by a Listen-Before-Talk (LBT) mechanism, while control and management signaling can be sent without LBT at maximum duty cycle of 5% within an observation period of 50 ms. The LBT is a functionality which means that the channel is assessed to be clear before transmission begins. Since CSI-RS can be considered a type of control/management signaling, LAA-LTE may send CSI-RS without LBT and may hence preserve the CSI-reporting schemes for TM10 UEs provided that the duty cycle of 5% is not exceeded. For example, transmission of CSI-RS may occur in a subframe every 20 ms.
The approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in the Background section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in the Background section.